The work extended our ongoing study of interactions between the retinal glial Muller cell and T lymphocytes. In an in vitro co- culture system, Muller cells had been shown to exert a profound inhibitory influence on antigen and IL-2 driven proliferation of T helper cell lines. Investigations of the nature of the inhibitory moiety revealed that it was sensitive to proteinase. In further studies, we demonstrated that Muller cells can produce interleukin 1 (IL-1) activity and that in conditions where their inhibitory action is removed they display the capacity to efficiently function as antigen presenting cells.